The power factor (PF) which can be achieved with an active switch-mode PFC controller is nearly 1 (i.e. 100%). The use of PFC controller, therefore, minimizes the power wasted in the harmonics and out-of-phase current.
A PFC converter is generally configured as a boost converter, with an inductor, a switch, and an energy delivery device. The energy delivery device is normally implemented as a diode or a MOS. The PFC controller controls on and off of the switch and diode to achieve unity power factor and to provide regulated DC output.
A PFC controller can be operated in continuous conduction mode (CCM), critical conduction mode (CRM), or discontinuous conduction mode (DCM). CRM and DCM modes are normally the choice for lower power consumption applications.
To provide load regulation for PFC output, a common method is used to sense the output voltage directly. The sensed output voltage is feedbacked to PFC controller to adjust the switch on/off timing. This conventional practice complicates the system design, and increases the board area and components.
Please refer to FIG. 1, which is a schematic diagram showing a conventional PFC switch-mode power supply having a load sense circuit to sense voltage at the converter output according to the prior art. In FIG. 1, the general configuration of the switch-mode power supply has a boost converter 11 and a PFC controller 12. The boost converter 11 is configured by an inductor 114, a resistor 117, an output stage 118, as well as an energy delivery device. The energy delivery device is usually implemented by a diode 115 or a transistor switch 116. The PFC controller 12 is configured by a AC voltage sense unit 121, an inductor current sense unit 122, a load sense unit 123, an arithmetic unit 124, a comparator 125 and a switch control unit 126. The PFC controller 12, through the load sense unit 123, senses the voltage from the load output stage 118 to control the on/off of the switch 116 and diode 115, realizes the effect promotion of the power factor, and provides a regulated DC voltage output.
Please refer to FIG. 2, which is a schematic diagram showing another conventional PFC switch-mode power supply having a load sensing circuit to sense voltage at the converter output according to the prior art. In FIG. 2, the general configuration of the switch-mode power supply has a boost converter 21 and a PFC controller 22. The boost converter 21 is configured by an inductor 214, an output stage 217, as well as an energy delivery device. The energy delivery device is usually implemented by a diode 215 or a transistor switch 216. The PFC controller 22 is configured by a AC voltage sense unit 221, an inductor current sense unit 222, a load sense unit 223, an arithmetic unit 224, a switch on-time unit 225, and a switch control unit 226. The PFC controller 22, through the load sense unit 223, senses the voltage from the load output stage 217 to control the on/off of the switch 216 and diode 215, realizes the effect promotion of the power factor, and provides a regulated DC voltage output.
Although a regulated output voltage can be provided by a conventional switch-mode power supply according to the prior art, it can only be realized by adding an additional sense unit. Therefore, the increases of the weight, components and the board area are still the inevitable defects of the prior art.
A solution of the above drawback in the prior art is not only to remove the voltage sense circuit from the load terminal of the switch-mode power supply but also to decrease the overall weight, components and the board area. Thus the invention of the case “switch-mode power supplies” would be the best way to solve the deficiencies of conventional means.